This invention relates to an apparatus for controlling air flow in a pressurized air line, by sampling the rate of air flow and blocking the line air flow passage whenever the flow rate exceeds a predetermined and selectable maximum. The invention also serves as a safety shut-off valve to block the air line whenever downstream air pressure drops below a minimum predetermined level. The invention is preferably used in conjunction with a system comprising a compressed air source and an air operated reciprocating pump, wherein the reciprocating pump is driven by means of a reciprocating air motor. Such an apparatus has the characteristic that the air motor delivers pumped fluid at a predetermined pressure and reciprocates at a rate required to maintain the predetermined pump fluid pressure. Therefore, whenever the pumped fluid is transferred, the air motor reciprocates only until fluid pressure is developed to the predetermined setting and then it ceases reciprocating. While the pumped fluid is being transferred, the air motor reciprocates at whatever rate is required to maintain the predetermined fluid pressure. Such a system has the disadvantage of causing the air motor to race or run away when the supply of pumped fluid becomes exhausted, or when the pumped fluid line suddenly breaks, thereby dropping fluid pressure to zero. In this case, the air motor reciprocates at an increasing rate to attempt to maintain the desired and predetermined pressure but, because it is impossible to maintain the desired fluid pressure, the air motor may reciprocate so fast that it becomes damaged or destroyed. Therefore, the present invention provides a safety valve to prevent such a runaway condition and to shut down the system whenever this condition occurs.
Prior art safety shut-off devices have been developed for venting or relieving a pressurized line whenever pressure exceeds a predetermined maximum. Prior art devices have also been developed for shutting off a pressurized air line whenever the downstream pressure drops below a predetermined value, as for example when a break occurs in a downstream pressure line. The present invention is an improvement over these prior art devices in that it provides a measure of safety control and shutoff under conditions less serious than an actual break in the pressure line, but which could result in damage or destruction of the air motor under the conditions hereinbefore described.